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<h2 class="title">
<a id="nested-objects"></a>嵌套对象<a class="edit_me edit_me_private" rel="nofollow" title="Editing on GitHub is available to Elastic" href="https://github.com/elasticsearch-cn/elasticsearch-definitive-guide/edit/cn/402_Nested/30_Nested_objects.asciidoc">edit</a>
</h2>
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<p>由于在 Elasticsearch 中单个文档的增删改都是原子性操作,那么将相关实体数据都存储在同一文档中也就理所当然。 比如说,我们可以将订单及其明细数据存储在一个文档中。又比如,我们可以将一篇博客文章的评论以一个 <code class="literal">comments</code> 数组的形式和博客文章放在一起：</p>
<div class="pre_wrapper lang-json">
<pre class="programlisting prettyprint lang-json">PUT /my_index/blogpost/1
{
  "title": "Nest eggs",
  "body":  "Making your money work...",
  "tags":  [ "cash", "shares" ],
  "comments": [ <a id="CO267-1"></a><i class="conum" data-value="1"></i>
    {
      "name":    "John Smith",
      "comment": "Great article",
      "age":     28,
      "stars":   4,
      "date":    "2014-09-01"
    },
    {
      "name":    "Alice White",
      "comment": "More like this please",
      "age":     31,
      "stars":   5,
      "date":    "2014-10-22"
    }
  ]
}</pre>
</div>
<div class="calloutlist">
<table border="0" summary="Callout list">
<tr>
<td align="left" valign="top" width="5%">
<p><a href="#CO267-1"><i class="conum" data-value="1"></i></a></p>
</td>
<td align="left" valign="top">
<p>如果我们依赖<a class="xref" href="dynamic-mapping.html" title="动态映射">字段自动映射</a>,那么 <code class="literal">comments</code> 字段会自动映射为 <code class="literal">object</code> 类型。</p>
</td>
</tr>
</table>
</div>
<p>由于所有的信息都在一个文档中,当我们查询时就没有必要去联合文章和评论文档,查询效率就很高。</p>
<p>但是当我们使用如下查询时,上面的文档也会被当做是符合条件的结果：</p>
<div class="pre_wrapper lang-json">
<pre class="programlisting prettyprint lang-json">GET /_search
{
  "query": {
    "bool": {
      "must": [
        { "match": { "name": "Alice" }},
        { "match": { "age":  28      }} <a id="CO268-1"></a><i class="conum" data-value="1"></i>
      ]
    }
  }
}</pre>
</div>
<div class="calloutlist">
<table border="0" summary="Callout list">
<tr>
<td align="left" valign="top" width="5%">
<p><a href="#CO268-1"><i class="conum" data-value="1"></i></a></p>
</td>
<td align="left" valign="top">
<p>Alice实际是31岁,不是28!</p>
</td>
</tr>
</table>
</div>
<p>正如我们在 <a class="xref" href="complex-core-fields.html#object-arrays" title="内部对象数组">对象数组</a> 中讨论的一样,出现上面这种问题的原因是 JSON 格式的文档被处理成如下的扁平式键值对的结构。</p>
<div class="pre_wrapper lang-json">
<pre class="programlisting prettyprint lang-json">{
  "title":            [ eggs, nest ],
  "body":             [ making, money, work, your ],
  "tags":             [ cash, shares ],
  "comments.name":    [ alice, john, smith, white ],
  "comments.comment": [ article, great, like, more, please, this ],
  "comments.age":     [ 28, 31 ],
  "comments.stars":   [ 4, 5 ],
  "comments.date":    [ 2014-09-01, 2014-10-22 ]
}</pre>
</div>
<p><code class="literal">Alice</code> 和 31 、 <code class="literal">John</code> 和 <code class="literal">2014-09-01</code> 之间的相关性信息不再存在。虽然 <code class="literal">object</code> 类型 (参见 <a class="xref" href="complex-core-fields.html#inner-objects" title="多层级对象">内部对象</a>) 在存储 <em>单一对象</em> 时非常有用,但对于对象数组的搜索而言,毫无用处。</p>
<p><em>嵌套对象</em> 就是来解决这个问题的。将 <code class="literal">comments</code> 字段类型设置为 <code class="literal">nested</code> 而不是 <code class="literal">object</code> 后,每一个嵌套对象都会被索引为一个 <em>隐藏的独立文档</em> ,举例如下:</p>
<div class="pre_wrapper lang-json">
<pre class="programlisting prettyprint lang-json">{ <a id="CO269-1"></a><i class="conum" data-value="1"></i>
  "comments.name":    [ john, smith ],
  "comments.comment": [ article, great ],
  "comments.age":     [ 28 ],
  "comments.stars":   [ 4 ],
  "comments.date":    [ 2014-09-01 ]
}
{ <a id="CO269-2"></a><i class="conum" data-value="2"></i>
  "comments.name":    [ alice, white ],
  "comments.comment": [ like, more, please, this ],
  "comments.age":     [ 31 ],
  "comments.stars":   [ 5 ],
  "comments.date":    [ 2014-10-22 ]
}
{ <a id="CO269-3"></a><i class="conum" data-value="3"></i>
  "title":            [ eggs, nest ],
  "body":             [ making, money, work, your ],
  "tags":             [ cash, shares ]
}</pre>
</div>
<div class="calloutlist">
<table border="0" summary="Callout list">
<tr>
<td align="left" valign="top" width="5%">
<p><a href="#CO269-1"><i class="conum" data-value="1"></i></a></p>
</td>
<td align="left" valign="top">
<p>第一个 <code class="literal">嵌套文档</code></p>
</td>
</tr>
<tr>
<td align="left" valign="top" width="5%">
<p><a href="#CO269-2"><i class="conum" data-value="2"></i></a></p>
</td>
<td align="left" valign="top">
<p>第二个 <code class="literal">嵌套文档</code></p>
</td>
</tr>
<tr>
<td align="left" valign="top" width="5%">
<p><a href="#CO269-3"><i class="conum" data-value="3"></i></a></p>
</td>
<td align="left" valign="top">
<p><em>根文档</em> 或者也可称为父文档</p>
</td>
</tr>
</table>
</div>
<p>在独立索引每一个嵌套对象后,对象中每个字段的相关性得以保留。我们查询时,也仅仅返回那些真正符合条件的文档。</p>
<p>不仅如此,由于嵌套文档直接存储在文档内部,查询时嵌套文档和根文档联合成本很低,速度和单独存储几乎一样。</p>
<p>嵌套文档是隐藏存储的,我们不能直接获取。如果要增删改一个嵌套对象,我们必须把整个文档重新索引才可以。值得注意的是,查询的时候返回的是整个文档,而不是嵌套文档本身。</p>




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